1. Field of the Invention
The present invention relates to an image formation apparatus which has an image masking control function for performing masking not to write an image in a non-image area, its image masking control method, and a recording medium in which an image masking control program has been recorded.
2. Related Background Art
Conventionally, an image formation apparatus such as an electrophotographic laser beam printer, a digital copying machine or the like has an image writing range which has been determined according to a sheet size. If image writing exceeds this image writing range, an image overreaches a sheet, toner adhered to the overreached part then adheres to a photosensitive drum, and the toner adhered to the drum further adheres to a transfer roller, whereby this roller is polluted. Such a state is explained in, e.g., Japanese Patent Application Laid-Open No. 2-226262 (referred as JPA '262 hereinafter) by the same assignee of the present application. However, JPA '262 merely describes that image overreaching in a sub-scanning direction, i.e., a sheet carrying direction, is prevented. For example, FIG. 2 of JPA '262 which is a timing chart of an enable signal ENBL represents that an image signal is passed only for a period of “true” of the signal ENBL by a signal masking an image signal VDO in the sub-scanning direction.
However, image overreaching occurs even in a main-scanning direction. If the image overreaching occurs in the main-scanning direction, the edge of a transfer roller 9 shown in FIG. 1 of JPA '262 is polluted by toner. When a next larger-sized sheet is fed, the toner adhered to the transfer roller 9 is then adhered to the back of the fed sheet, thereby causing a phenomenon called “a back stain”. Especially, when printing is performed on both faces of the sheet, this back stain always causes serious image pollution.
In recent years, a user's demand to effectively use a sheet by writing an image to the marginal edge of the sheet becomes strong. For this reason, it is necessary to prevent, by performing image masking of high accuracy more than the past, an image writing overrun accurately at the marginal edge of the sheet if an image overreaches an image area on this sheet.
Further, a case where plural laser beams are used to write a color image is explained in, e.g., Japanese Patent Application Laid-Open No. 57-67375 by the same assignee of the present application. In this case, misregister (or deviation) of each beam makes accurate high-quality masking difficult.
Further, recently, a multifunctional information output apparatus called a multifunctional printer (MFP) has been in demand. In this MFP, an image scanner is incorporated with a printer, and an image read by the image scanner is output to the printer as it is. When an original is read by the image scanner, if its position is dislocated or its size is small, the periphery of this original is read as black. Thus, in a case where an image signal representing the black-periphery original is printed as it is, toner is not transferred to a sheet but is adhered to a transfer roller when a sheet position is slightly dislocated, thereby causing a back stain of a next sheet. Also, when the original is slightly dislocated, the black part on the periphery of the original is easily read by the image scanner.